Not applicable.
1. Field of the Invention
This invention relates to current confinement and isolation in vertical cavity surface emitting lasers.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer""s surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics. In particular, the various material systems can be tailored to produce different laser wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and so on.
VCSELs usually include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, spacers, current confinement structures, substrates, and contacts. Because of their relatively complicated structure and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAS) or an indium phosphorus (InP) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type graded index top spacer 22 is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As an optical cavity is resonant only at specific wavelengths, the mirror separation is controlled to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that provides current confinement. The insulating region 40 is usually formed either by implanting protons into the top mirror stack 24, or by forming an oxide layer. Either way, the insulating region 40 defines a conductive annular central opening 42. Thus, the central opening 42 forms an electrically conductive path though the insulating region 40 to the active region.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that it flows through the conductive central opening 42 and into the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical VCSEL, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added.
While generally successful, VCSELs have problems. For example, in some material systems producing an insulating region 40 using either proton implantation or oxide isolation is not practical. Implanted ions do not produce suitable trap levels while oxide layers are difficult to implement. These problems are particularly apparent in long wavelength VCSELs, and especially in long wavelength VCSELs arrays. Not only do long wavelength VCSELs require current confinement, but low capacitance and easy manufacturing are also required. With VCSEL arrays it is important to isolate the individual VCSEL elements. Typically, current confinement and isolation in long wavelength VCSELs are produced using either a patterned tunnel junction, which has high capacitance, or an undercut etch, which does not lend itself to easy manufacturing.
The transition metals and oxygen are good sources of deep traps that are not related to the implant damage, but are related to the element themselves. In particular, Cr, and Fe have been used to dope during growths of GaAs and InP substrates and epilayers so as to obtain semi-insulating behavior. Cr is used more in GaAs while Fe more in InP. Oxygen has been implanted in several kind of devices, including GaAs based VCSELs, for isolation and current guiding. However, it has not been used in InP based VCSELs.
Therefore, a VCSEL having a new current confinement structure would be beneficial. Even more beneficial would be a new current confinement structure that has low capacitance and that is easy to manufacture. Also beneficial would be a new current confinement structure that has low capacitance, that is easy to manufacture, and that provides good isolation between VCSEL elements in a VCSEL array.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
Accordingly, the principles of the present invention are directed to a VCSEL having a new current confinement structure. Those principles further provide for a current confinement structure that has low capacitance and that is easy to manufacture. A current confinement structures according to the principles of the present invention can also provide good isolation between VCSEL elements in a VCSEL array.
A VCSEL according to the principles of the present invention includes a current confinement structure formed from deep traps in a top DBR mirror and/or in a top spacer. Isolation between VCSEL elements in a VCSEL array can be provided by extending the deep traps into an active layer. The deep traps are produced by implanting transition metals, for example Fe or Cr into a III-V substrate (such as Inp or GaAs). The energy and dosage used when implanting can be tailored to control the lateral sheet resistance and/or the isolation. Furthermore, the implants beneficially extend over a significant depth, thereby reducing the capacitance across the resulting VCSEL.
The elements that are suitable for implantation include Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Tc, Ru, Rh, Pd Ag, Hf, Ta, W, Re, Os, Ir, Pt, Au. The preferred subset (because of their abundance and known characteristics) includes Ti, V, Cr, Mn, Fe, Co, Ni. In addition, oxygen is well suited to implantation. Elements which may work include numbers 58-71, and 90-103.
Beneficially, after implanting deep elemental traps to make a material semi-insulating, an implant anneal is performed. Such annealing is typically performed between 700 and 950 C., with a longer annealing time being used at lower temperatures.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.